For a battery case of an enclosed nickel metal hydride storage battery, a resin battery case or a metal battery case is used. Furthermore, a battery case whose surface is coated with a metal layer has been proposed (see, for example, Japanese Patent No. 3049854).
An enclosed nickel metal hydride storage battery is designed so that the capacity of the negative electrode is larger than the capacity of the positive electrode. Excess capacity that can be charged of the negative electrode is referred to as “discharge reserve,” and excess capacity of the negative electrode that can be discharged is referred to as “discharge reserve.” The discharge capacity of the battery is limited by the capacity of the positive electrode (positive electrode regulation). By making the negative electrode capacity be larger than the positive electrode capacity, it is possible to suppress the increase in the internal pressure at the time of overcharging and overdischarging.
In a nickel metal hydride storage battery using a resin battery case, a small amount of hydrogen continues to leak out through the battery case. If hydrogen leaks out from the battery, the discharge reserve at the negative electrode is decreased in accordance with the amount of leaked hydrogen. Therefore, in the nickel metal hydride storage battery that was used for a long time, the capacity balance between the positive electrode and the negative electrode is lost and the negative electrode capacity is decreased. As a result, the discharge capacity may be decreased or oxygen may be generated from the negative electrode at the time of overdischarging. Thus, it is thought that in such a nickel metal hydride storage battery using a resin battery case, a so-called long-term deterioration that appears when a small amount of hydrogen continues to leak out for a long time. Since hydrogen leaks out very slowly, it does not matter during a general using term. However, this deterioration may matter in batteries that are intended to be used for a long time, for example, batteries that are required to have 10 years or more of life.
On the other hand, in a metal battery case or a resin battery case coated with a metal layer, since hydrogen is not likely to permeate the battery case, the above-mentioned deterioration is not likely to occur. However, in the nickel metal hydride storage battery, regardless of materials of the battery case, a hydrogen-absorbing alloy at the negative electrode is corroded and a side reaction occurs in which the amount of hydrogen to be absorbed by the hydrogen-absorbing alloy is increased. As a result, the discharge reserve at the negative electrode is increased while the charge reserve at the negative electrode is decreased and thus the internal pressure of the battery is increased easily at the time of charging. When the charge reserve continues to be decreased due to a long period of use, the internal pressure of the battery becomes too high at the time of full charging and the safety valve operates. As a result, gas or electrolyte inside the battery is released, which may deteriorate the property of the battery. Thus, it is thought that in the nickel metal hydride storage battery, regardless of materials of the battery case, long-term deterioration may occur due to the corrosion of the hydrogen absorbing alloy after it is used for a long time.